There exist devices of this type in various designs. One applicable type, which is currently the object of intensive development, is the so-called implantable retina prosthetic or retina implant. They usually include an electrode array, which can be implanted in the subretina or epiretina in active contact with the retinal tissue that can be electrically stimulated. Said electrode array is supposed to compensate at least to a certain degree for the degeneration of the retinal photoreceptors that leads to blindness. That is, the electrode array is supposed to replace the function of the retinal photoreceptors with a space-resolved electric stimulation of the neural cells in the retina.
The most common of these devices is the unipolar stimulating electrode array, also called the monopolar type, where an electrode array chip carries a two dimensional arrangement of individual stimulation electrodes on a front side, which in the implanted state faces the retinal tissue that can be stimulated. These stimulation electrodes are assigned a planar counter-electrode at a relatively far distance in front of the front side or on the back side of the chip—that is, at a distance that is very much larger than the distance between the adjacent stimulation electrodes. A suitable medium can be used to apply a stimulation signal to the stimulation electrodes. This stimulation signal is usually a pulsed voltage signal, by means of which said stimulation electrodes generate a stimulation field—usually in the form of electric currents between the stimulation electrodes, on the one hand, and the counter-electrode, on the other hand—in the adjacent retinal tissue that can be stimulated. In a passive array type the energy required for stimulation is taken solely from the incident visible light by means of, for example, a photodiode layer sequence in the array chip. In order to achieve higher stimulation currents with simultaneously optimal spatial resolution and consequently with miniature stimulation electrodes, additional energy is fed, for example, into an active array type by means of pulsed infrared radiation. For this and other details about unipolar electrode arrays reference is made to the pertinent literature. See also the published patent applications (Offenlegungsschrift) DE 101 51 650 A1 and DE 103 29 615 A1 as well as the patent DE 197 05 987 C2.
In order to achieve simultaneously a satisfactory spatial resolution and response sensitivity for such retina implants, in principle an electrode array with a very dense arrangement of single micro-electrodes, which are capable of delivering a high stimulation field—that is, stimulation currents that exceed an excitation threshold value of the retinal cells—, and of acquiring a large stimulation volume, in which the stimulation field exceeds the excitation threshold value of the retinal cells, is desirable. On the other hand, stimulation fields that are too high locally and that could lead to tissue damage must be avoided.
At this point the conventional unipolar electrode arrays appear to reach their limit. The fact that only transient currents are observed when a voltage is applied to the stimulation electrodes is ascribed to the formation of a Helmholtz double layer at the interface between the electrode array and the tissue of the retina, even though the active mechanism of this phenomenon has not been explained in detail yet. In order to achieve higher transient currents the literature suggests a porous design of the stimulation electrodes with suitably elevated electrode surfaces. See, for example, the journal article by A. Stett and H. Hämmerle, “Subretinal Prosthetics for the Blind”, Bulletin SEV/VSE March 1999, p. 11. Such research is not very likely to support the observation of the electrode array—retinal tissue—interface as a purely electric capacitor. Rather it is more likely to point to the “battery effect” on account of the electrolytic properties of the tissue. If the stimulation electrodes are packed more densely and the electric current density is higher, a collective current effect can also be expected with unipolar electrode arrays. Such an effect can lead to undesired high voltage drops and to the dependency of the voltage to be applied for stimulation on the number of stimulation electrodes that are active—that is, whether the visual environment is bright or dark.
As an alternative to the monopolar electrode array, one has also already experimented with bipolar electrode arrays, where front-sided counter-electrodes have been assigned individually to front-sided stimulation electrodes. Since the propagation of the electric current is locally restricted in the tissue of the retina, such a bipolar arrangement has been attributed the ability to generate, as compared to the monopolar type, more intensive voltage drops in the area between the electrodes (as stated by A. Stett and H. Hämmerle in the above-cited journal article). The journal article by J. F. Rizzo III et al., “Methods and perceptual thresholds for short-term electrical stimulation of the human retina with micro-electrode arrays”, Investigative Opthalmology & Visual Science, December 2003, volume 44, no. 12, p. 5355, discloses not only monopolar electrode arrays, but also a bipolar electrode array with circular front-sided stimulation electrodes, which are separated from a front-sided contiguous counter-electrode by means of an annular gap.
The invention is based on the technical problem of providing a device of the type that is described in the introduction and with which even in the event of a high electrode integration density a biological material that can be stimulated can be electrically stimulated with comparatively high stimulation efficiency by using an implantable electrode array.
The invention solves this problem by providing a device with the features of claim 1. This device is designed in such a manner that the alternating field stimulation signals are applied to the individual electrodes of the electrode array, which can be implanted in active contact with the biological material that can be electrically stimulated; that the electrode array forms at least two tripolar or higher multipolar multipole elementary cells from three or more adjacent single electrodes. The stimulation field, which is produced from each multipole elementary cell for the biological material and which may have the form of electric stimulation currents, exhibits a rotation component; and at least one of the individual electrodes belongs to at least two multipole elementary cells. Hence, it is possible to produce a total stimulation field from the individual multipole fields of the elementary cells with a rotating field component. The field components of every two adjacent multipole elementary cells may rotate, for example, simultaneously and counter-clockwise.
Owing to the “lighthouse effect” of the rotation, the rotation of the stimulation field makes possible a significant increase in the probability that a stimulable cell in the adjacent biological material is “hit” by a local stimulation vector that exceeds the cell's excitation threshold value and is, thus, actually stimulated. At the same time it must be taken into consideration that especially in retinal application the stimulable cells in the biological material are generally arranged in a relatively random manner in a network; and in addition, their excitation threshold value is usually direction-dependent. Thus, in the case of cylindrical/rod-shaped axons and dentrites of the retina it is observed that they are stimulated more easily parallel to their cylindrical direction (that is, with a lower excitation threshold value) than in the direction orthogonal thereto. Rotating the stimulation field enhances the probability that a stimulable cell will be “hit” by an adequately high stimulation field vector even in a worse stimulation direction or at least will still be hit by a stimulation field vector in a good stimulation direction with a then lower excitation threshold value. In both cases the cell is then stimulated, whereas in the case of a non-rotating stimulation field the cell would not be hit with an adequately large excitation field vector and would, therefore, not be simulated.
In other words, the inventive rotation of the stimulation field enhances the stimulation efficiency and increases the stimulation volume, i.e., the volume of the biological material that can be stimulated by the stimulation field, a feature that is very important especially for very tight electrode spacing. Depending on the application, other field components—for example, a time-pulsing component and/or an oscillation component, i.e., a component that oscillates in two opposing directions—can be superimposed on the rotation component.
In an advantageous further development of the invention, the three or more adjacent, corresponding multipole elementary cells (that is, single electrodes that are coupled as a tripole, quadrupole, etc.), are actuated with the alternating field stimulation signal in such a manner that the stimulation field that said electrodes jointly produce in the biological material exhibits a field component that rotates about a multipole axis. This multipole actuation of the individual electrodes is an especially simple to realize and effective possibility for producing a stimulation field with a rotation component. Therefore, in another design of the invention a stimulation field can be produced with a field component, which rotates at a constant angular velocity, by applying alternating voltage signals, which are suitably phase shifted with respect to each other, to the individual electrodes of the multipole. At the same time an oscillation component may be generated. By coupling the individual electrodes to one or more multipoles a correspondingly multipolar electrode array is formed, in which each individual electrode of the multipole(s) acts as both a stimulation electrode and as a counter-electrode in the conventional sense. That is, the individual electrodes may all be located on the front side of the electrode array that in the implanted state faces the biological material; and the at least tripolar electrode array forms a higher polar expansion of the aforementioned monopolar and bipolar electrode array.
In a further development of the invention, the individual electrodes of the electrode array form a preferably regular electrode lattice structure comprising a plurality of multipole elementary cells that lie side-by-side and that comprise at least three single electrodes respectively. Each internal electrode of the lattice belongs to a plurality of multipole elementary cells.
In another design of the invention, the individual electrodes form, for example, a regular triangular lattice, in which every three adjacent single electrodes may be coupled to form a tripole; or a regular rectangular lattice, in which every four adjacent single electrodes may be coupled to form a quadrupole. The generated stimulation fields of every two adjacent multipoles may exhibit components that rotate simultaneously counter-clockwise.
The design of the inventive measures to apply phase-shifted alternating voltage signals to the individual electrodes of a multipole elementary cell takes into consideration an image information component and a signal shape component as additional factors that can be chosen on an individual basis for the individual electrodes. The image information component includes the image information to be perceived; and the signal shape component includes additional signal conditioning measures, such as a pulsed drive with predefined pulse parameters.
In an especially advantageous embodiment, the invention provides a retina implantable micro-electrode array, in which all of the individual electrodes are disposed on one side of a carrier chip and are actuated in a coupled manner in one of the aforementioned ways as tripoles or higher multipoles, so that a multipole stimulation field with rotating field contributions of the individual multipoles for stimulating the retina is produced. At the same time the field contributions of the adjacent multipoles may rotate simultaneously and counter-clockwise.
Advantageous embodiments of the invention are depicted in the drawings and are described below.